The field of this invention is electronic circuitry for producing an output voltage which varies in magnitude and polarity with the frequency of an applied signal. Such circuits are known as frequency discriminators and are widely used in such applications as frequency modulation (fm) receivers, radar sets and frequency meters. Well known types of discriminators are the Foster-Seeley discriminator and the ratio detector.
In the past it has been known that fm signals can be received on amplitude modulation (am) receivers by detuning the fm carrier to one of the skirts (or the sloping portion) of the receiver'intermediate frequency stages. In this way the frequency modulation is converted into amplitude modulation which is then detected by the receiver's detector. A more sophisticated frequency discriminator utilizing this principle has also been known in the prior art. This prior art discriminator comprises a pair of parallel resonant circuits, one tuned above the center frequency and the other an equal amount below it. The input frequency is applied in parallel to both tuned circuits and the outputs of both circuits are separately rectified by means of oppositely polarized diodes, and the rectified voltages combined to form the discriminator output.
The present invention is a frequency discriminator which operates on a completely different principle from any known prior art discriminator and which has important advantages over prior circuits of this type. The novel discriminator includes a pair of tuned circuits both tuned to the same frequency, the center frequency. One of these circuits is a frequency pass circuit and the other a frequency stop circuit. The input frequency is split into two channels and differentially phase shifted by 90.degree. before application to the two tuned circuits. The phase shifts undergone by the signals in the two tuned circuits are such that above the center frequency the outputs of the two tuned circuits are always co-phasal, and below the center frequency they are always 180.degree. out of phase. Thus the product (or dc component) of these tuned circuit outputs will be negative below the center frequency and positive above it, or vice versa. Further, the output of one of these circuits is zero at the center frequency and the output of the other approaches zero at zero frequency and at infinitely high frequency. The tuned circuit outputs are multiplied by one another and the product will have an S-shaped curve which is a necessary characteristic of a discriminator.